Taeniodonta

The Taeniodonta—from Greek roots meaning "ribbon tooth," in allusion to the restricted band of enamel on the canines and cheek teeth of specialized forms—constitute a small and peculiarly specialized order of mammals whose broader affinities appear to lie with Cimolesta. The bizarre morphology of specialized taeniodonts has long obscured their relationships, but the most primitive representatives are said to share tarsal and/or dental traits with cimolestids and pantolestids (Szalay, 1977; Schoch, 1986), suggesting affinities with the Cimolesta. This hypothesis was strengthened when Eberle (1999) proposed that the large, bunodont cimolestid Alveugena (Fig. 7.15A), from the early Paleocene (Puercan) of Wyoming, could be the closest relative and possible ancestor of taeniodonts. Unfortunately, it is known only from skull fragments and the upper dentition, making comprehensive comparison impossible. The recent discovery of a possible Late Cretaceous taeniodont (discussed below) further complicates the issue of taeniodont origins and casts doubt on a direct relationship between Alveugena and taeniodonts. Earlier suggestions of resemblance and relationship to edentates or tillodonts, however, are now considered superficial and convergent.

Taeniodonts are known principally from the early Paleo-cene through middle Eocene of North America. In addition, two records from the early Eocene of Europe have been considered to represent taeniodonts; they are reviewed below. The North American forms comprise ten genera in two families (or two subfamilies of a single family), Conorycti-dae and Stylinodontidae.

Taeniodonts have rather simple, tritubercular molars with low, bunodont cusps that were quickly obliterated by heavy wear (Fig. 7.15) and hence are preserved only in young individuals. The upper molars are transversely narrower than those of many other cimolestans. They have a reduced sty-lar shelf, no pre- or postcingula, and no hypocone. The para-cone and metacone are separate and very buccally situated, and the protocone and conules are low and very lingual. The lower molars also lack cingula, and the trigonids and talonids are typically more or less equal in size and height. The molars decrease in size posteriorly. The skeleton of taeniodonts primitively was relatively robust, and became progressively more so during their evolution.

Several morphologic trends occurred in taeniodont evolution, leading to the distinctive or unique specializations of Eocene stylinodontids (Patterson, 1949; Schoch, 1986; Lucas et al., 1998). There was increasing extension of the enamel on the buccal side of lower cheek teeth and on the lingual side of uppers, a condition known as crown hypso-donty This condition led to elongated roots and, ultimately, hypselodonty (ever-growing teeth with open roots). The canine teeth in particular underwent progressive hypertrophy and hypselodonty as well. These trends were presumably a response to an increasingly abrasive diet, perhaps associated with an unusual foraging behavior. The skull and mandible of taeniodonts (Fig. 7.16) became shorter and deeper through time, and the skeleton became conspicuously more robust. At the same time, taeniodonts were increasing in body size. Several of these trends occurred in parallel in the two families.

The most primitive taeniodonts are assigned to the para-phyletic family Conoryctidae (Schoch, 1986; Lucas et al., 1998). They were small to medium-sized (5-15 kg) generalized omnivores restricted to the early Paleocene; Conoryctes, the latest occurring representative, became extinct at the end of the Torrejonian (Lofgren et al., 2004). In the most primitive conoryctid, Onychodectes (Figs. 7.15B,C, 7.16C), the skull was rather long and narrow, with confluent orbital and temporal fossae, and a shallow dentary. The molars already show heavy wear, indicating a proclivity for an abrasive diet. The canine was prominent, the incisors small, and

the premolars simple. The dental formula, 3.1.4.3/3.1.4.3, remained the same in most taeniodonts except for the tendency to reduce the number of incisors. Onychodectes and Conoryctella retained three lower incisors, whereas Cono-ryctes may have had only two.

The limbs of Onychodectes (Fig. 7.17) were generalized in proportion, but somewhat robust, suggesting climbing or digging capabilities. The humerus is broad distally and has a prominent deltopectoral crest, and the ulnar olecranon is pronounced. The manus and pes are pentadactyl, with small claws. There was a long and well-developed tail. Apart from being a little larger and more robust, Onychodectes was similar in body form, and perhaps habits, to the opossum Didel-phis (Schoch, 1986).

Stylinodontidae were already divergent from Conorycti-dae in the early Paleocene (middle Puercan) and possibly

Fig. 7.14. Convergence of the manus in the apatemyid Heterohyus, the phalangeroid marsupial Dactylopsila, and the lemuroid primate Daubentonia. (From Koenigswald, 1990.)

Fig. 7.15. Dentitions of taeniodonts and a possibly related cimolestid (anterior to left): (A) left upper teeth of the Puercan cimolestid Alveugena; (B) skull of early Paleocene Onychodectes (teeth typically worn); (C) Onychodectes, left upper and right lower teeth (unworn); (D) early Paleocene Wortmania, left upper and right lower teeth; (E) late Paleocene-early Eocene Ectoganus, left upper and right lower teeth; (F) Eocene Stylinodon, left upper and right lower teeth. (A from Eberle, 1999; B-F from Schoch, 1986.)

Fig. 7.15. Dentitions of taeniodonts and a possibly related cimolestid (anterior to left): (A) left upper teeth of the Puercan cimolestid Alveugena; (B) skull of early Paleocene Onychodectes (teeth typically worn); (C) Onychodectes, left upper and right lower teeth (unworn); (D) early Paleocene Wortmania, left upper and right lower teeth; (E) late Paleocene-early Eocene Ectoganus, left upper and right lower teeth; (F) Eocene Stylinodon, left upper and right lower teeth. (A from Eberle, 1999; B-F from Schoch, 1986.)

before (see below). Most were larger and more robust than conoryctids (10-110 kg; Schoch, 1986; Lucas et al., 1998). Early Paleocene Wortmania (Figs. 7.15D, 7.16B), the oldest unequivocal member of the family, is more derived than conoryctids in having a somewhat deeper skull and mandible, only one incisor in each quadrant, larger canines, and transversely oriented lower premolars—all hallmarks of the family. The coronoid process of the mandible was noticeably larger than in conoryctids, suggesting larger temporalis muscles. The skeleton was markedly more robust, and there were large, recurved claws on the manus.

These traits were further accentuated in other stylino-dontids. The skull and mandible became shorter and deeper, especially the symphysis, which was solidly fused. The sagittal crest increased in prominence, and the occiput broadened. In Paleocene stylinodontids such as Psittacotherium (Fig. 7.16A), the canines were very large, but not yet evergrowing. In Eocene Ectoganus and Stylinodon (Fig. 7.15E,F), however, they became huge, ever-growing, and gliriform, with enamel restricted to the labial surface, superficially resembling the enlarged incisors of rodents and trogosine tillodonts. In similar fashion to rodents, this morphology maintained a sharp, chisel-like cutting edge on the canines as the softer dentine wore down behind the hard enamel border. At the same time, the crowns of the cheek teeth became progressively more hypsodont. In the most derived taeniodont, middle Eocene Stylinodon, all teeth were evergrowing, and the posterior premolars and molars were reduced by wear to cylindrical dentine pegs with buccal and lingual bands of enamel. Unworn molars, however, show a bilophodont crown pattern.

Stylinodontids had massive skeletons (Fig. 7.18). The limb elements are short and stout: the humerus is very broad distally and has enormous supinator and delto-pectoral crests, the ulnar olecranon is very prominent, and the radius is much shorter than the humerus. The bones of the manus and pes are also short and stout, and include numerous large flexor sesamoids in the manus (Turnbull, 2004). The digits of the manus bore particularly large, curved claws, which were long presumed to have been used for

burrowing and digging up subterranean food items. Thus Schoch (1986: 1) concluded that derived stylinodonts were proficient rooters and grubbers, analogous to "an aardvark, with the head of a pig." More recently, however, Turnbull (2004) observed that Stylinodon lacks the abrasive scratches to be expected in a rooter. Instead he hypothesized that it used its powerful forelimbs to hook vines and branches and pull them through the teeth to strip off leaves and fruit. Whether such a diet could explain the hyselodonty in Stylinodon, not to mention its extremely robust limb elements, is conjectural.

An analysis of taeniodont tooth wear by scanning electron microscopy might help to resolve this controversy.

A new North American taeniodont, Schowalteria (Fig. 7.16D), was recently described from the latest Cretaceous (Lancian) of Alberta, Canada (Fox and Naylor, 2003). It is based on a snout and dentary, similar in size to its contemporaries Cimolestes magnus and Didelphodon vorax and is thus smaller than other taeniodonts. The cheek teeth are very heavily worn, obscuring most surface details, but a narrow stylar shelf and shallow ectoflexus (indentation of the buc-

cal margin) are evident. According to Fox and Naylor, several features—including the robust canines with restricted enamel; enlarged central incisor (interpreted as I2, Ij being absent); short, deep snout; and structure of the zygomatic arch—suggest that Schowalteria is related not just to taenio-donts, but specifically to stylinodontids. If their interpretation is correct, it challenges the hypothesis that Alveugena is the sister taxon of Taeniodonta. Nonetheless, no group other than cimolestids has been advanced as a likely relative. Thus the existence of taeniodonts in the Late Cretaceous might be indirect evidence of unknown older cimolestids. Although Schowalteria appears to be more plesiomorphic than other taeniodonts in nearly all dental features, it is already more derived than primitive taeniodonts in lacking Ir This feature raises the possibility that its taeniodont-like features could be convergent.

Taeniodonts may well have been restricted to North America. Their basic interrelationships are shown in Fig. 7.19B. The only non-North American taxa that have been assigned to the order are early Eocene Eurodon and Less-nessina from Europe, which have been interpreted as cono-ryctids (Estravis and Russell, 1992). They are represented so far only by a few dental specimens, which are low crowned, bunodont, and very small. Although these specimens share certain features with primitive conoryctids such as Ony-chodectes, they contrast with taeniodonts in other features (the presence of strong cingula on upper molars, expanded talonid on M3) that are reminiscent of condylarths, such as

Fig. 7.19. (A) Relationships of tillodonts and pantodonts; dashed lines indicate tentative positions. (B) Relationships of taeniodonts. The position of Late Cretaceous Schowalteria (not shown) is uncertain; it is more primitive in some features than the basal conoryctid Onychodectes, but in other features it appears to be a basal stylinodontid (Fox and Naylor, 2003). (A based mainly on Muizon and Marshall, 1992, and Lucas, 1993, 1998; B mainly after Schoch, 1986, and Eberle, 1999.)

Fig. 7.19. (A) Relationships of tillodonts and pantodonts; dashed lines indicate tentative positions. (B) Relationships of taeniodonts. The position of Late Cretaceous Schowalteria (not shown) is uncertain; it is more primitive in some features than the basal conoryctid Onychodectes, but in other features it appears to be a basal stylinodontid (Fox and Naylor, 2003). (A based mainly on Muizon and Marshall, 1992, and Lucas, 1993, 1998; B mainly after Schoch, 1986, and Eberle, 1999.)

hyopsodontids. Lessnessina, in fact, was initially identified as a periptychid condylarth (Hooker, 1979) and most recently was considered to be a hyopsodontid (Hooker and Dash-zeveg, 2003). The relationship of these mammals to taeniodonts should be considered questionable until more definitive evidence is known.